European Patent Application No. 0085444 describes a device which is capable of being used for straight drilling or for drilling at an angle. In particular the application describes a method and means for controlling the course of a bore hole during drilling.
The method and means includes first and second stabilisers which are arranged to support the housing for a down-hole motor having an output shaft for connecting to a drill bit. At least one of the stabilisers is eccentric relative to the housing so that rotation of the housing will cause a change in the angle of the axis of the output shaft of the down-hole motor. Thus by controlling the rotation of the housing and the length of time of operation of the down-hole motor the course of the borehole can be controlled.
The change in angle causes stresses to be introduced into the housing which are transmitted to the drill bit causing excessive friction between the drill bit and the wall of the bore-hole. Furthermore additional strains are imposed on the stabilisers, the connections between the drill string and the housing, between the down-hole motor output shaft and the drill bit and between sections of the housing. These stresses can lead to damage and/or excessive wear of bearings.
It is an object of the present invention to provide an improved device for use in underground drilling which reduces the stress-related problems identified above.